The present invention is directed to a method for determining measured values in an arrangement including a plurality of optical fiber Bragg grating sensors, as well as to an arrangement for implementing this method.
Optical fiber Bragg grating sensors (FBGs) are a new type of very rugged class of sensors used for measuring strains, preferably highly resolved to measuring lengths of 0.1 to 20 mm, or also temperature differences.
In aerospace and aeronautics, as well as in the automotive and railroad sectors, these sensors open up many potential applications in the area of vehicle inspection, particularly for all types of adaptive control systems, and, in the metrology sector, for example for experimental stress analysis. Optical fiber Bragg grating sensors of this kind are known, for example, from U.S. Pat. No. 4,996,419. Compared to conventional approaches employing strain gauges or piezo-sensory technology, Bragg grating sensors offer a number of benefits due to their compact type of construction. Their small fiber diameter, in particular, makes them especially suited for integration in fiber-reinforced composite structures, as frequently used in aeronautic construction. Since the measuring signal of the optical fiber Bragg grating sensors is frequency-specific and not amplitude-specific, the measuring signal proves to be independent of the transmitted distance and, thus, also independent of power fluctuations caused by the transmission line. Moreover, as all optical fiber sensors, this sensor proves to be insensitive to electromagnetic interference.
From U.S. Pat. No. 5,401,956, describes a measuring system which is based on Bragg grating sensors and can be used to detect pressure or temperature influences.
From German Patent Application No. DE 198 56 549 of the applicant, which is not believed to be prior art to the present application, it is known to combine a plurality of optical fiber Bragg grating sensors exhibiting different frequency selectivities into an array; to irradiate them, together with a wavelength-tunable laser, with a light signal that is variable in wavelength; to analyze the response signal that is sensor-selective due to the frequency-selective formation of the various Bragg grating sensors of the array; and to thereby provide information about the measured value in terms of the specific, selected Bragg grating sensor. Thus, measured values, i.e., strain, temperature or measured pressure values, can be determined in one arrangement at the location of a specific sensor of an array.
The potential number of sensors that can be used in an array of this kind is limited by the spectral width of the laser and by the maximal strain of the Bragg grating sensors. For example, using the best tunable lasers, free of mode hopping, that are currently known, it is possible to tune only one spectral range of about 100 nm. This means that, under favorable conditions, a maximum of 100 sensors can be accommodated and, under unfavorable conditions, not even 10 sensors can be accommodated in the array, without losing the ability to distinguish among the sensor signals.
An object of the present invention is to provide an arrangement and a method for determining measured values in such an arrangement using a plurality of optical fiber Bragg grating sensors, which will make it possible to provide an increased number of sensors and, as a result, a location-specific analysis of measuring results.
The present invention provides a method for determining measured values in an arrangement (3) comprised of a plurality of optical fiber Bragg grating sensors (FBGs) (11), which are combined to form various arrays of optical fiber Bragg grating sensors (11), which are arranged one behind the other, are connected by fiber optics, and exhibit different frequency selectivities, and of delay elements (12), which are assigned to these arrays, the optical fiber Bragg grating sensors (11) receiving a query signal which represents temporally differentiated, frequency-selective light, a response signal being evaluated, in that individual partial signals of the response signal are specifically assigned to one or a few optical fiber Bragg grating sensors (FBGs) (11) on the basis of their specific frequency and on the basis of their propagation delay that is essentially determined by the delay elements (12).
The present invention also provides an arrangement comprised of a plurality of optical fiber Bragg grating sensors (FBGs) (11) for implementing the method according to the present invention for determining measured values, where optical fiber Bragg grating sensors (FBGs) are combined to form various arrays of optical fiber Bragg grating sensors (11) that are arranged one behind the other, connected by fiber optics, and have different frequency selectivities, and where delay elements (12) are assigned to these arrays in such a way that the optical fiber Bragg grating sensors (11) are able to receive a query signal which represents temporally differentiated, frequency-selective light, and a response signal is evaluated in such a way that individual partial signals of the response signal are able to be specifically assigned to one or a few optical fiber Bragg grating sensors (FBGs) (11) on the basis of their specific frequency and on the basis of their propagation delay that is essentially determined by the delay elements (12).
In accordance with the present invention, the arrangement composed of a plurality of optical fiber Bragg grating sensors shows a plurality of arrays, which are constituted of individual, optical fiber Bragg grating sensors that are connected by fiber optics, configured one behind the other, and have different frequency selectivity. The various arrays are linked to delay elements, formed at regular intervals by delay lines. The optical fiber Bragg grating sensors receive light from a light source, constituted, for example, of a tunable laser or LED. As a function of the specific frequency selectivities of the individual Bragg grating sensors, specific sensors are activated by the selected wavelength of the light source and transmit a corresponding response signal. The other sensors function passively in response to this wavelength. To distinguish among the activated Bragg grating sensors of the various arrays, these various arrays are connected to delay elements, with the result that the various individual response signals are fed at different times to the receiving system of the measuring arrangement, for example for strain values, temperature values or pressure values. With knowledge of the specific delay times and, thus, of the expected propagation delays of the response signals of the individual sensors of the array, these signals are then able to be differentiated by the receiving system. This ability to distinguish among the individual sensors and, thus, the measured values at the sensor location from others, allows a very differentiated assertion to be made with respect to the measured value distribution over a predefined space having a multiplicity of connected Bragg grating sensors. This arrangement in accordance with the present invention and the method for determining measured values in accordance with the present invention make it possible to distinguish among a multiplicity of differentiated measured values by the use of a corresponding multiplicity of differentiated Bragg grating sensors and to selectively analyze them accordingly. For example, employing this arrangement and method, it is possible to mount several hundred Bragg grating sensors, which are driven and connected according to the present invention, on an airplane wing and, during a brief flight simulation, to measure the strain values of the wing at the positions of the multiplicity of Bragg grating sensors and to analyze them as spatially resolved values.
From the frequency-selective light source that is controlled as a time-differentiated source, for example in the form of a pulsed, spectrally tunable laser that is controlled temporally as well as spectrally by a central control unit, the Bragg grating sensors receive substantially identical light pulses. These light pulses are essentially distinguished from one another only by the time shifts caused by the delay elements that they pass through, and they are obtained, in particular, in that one or a plurality of couplers split the light pulses into a plurality of pulses, all of the same wavelength. In response to the Bragg grating sensors receiving the query signal made up of substantially identical pulses, partial signals are formed in the Bragg grating sensors and generate the arrangement""s response signal that is fed to a receiver. The receiver is preferably connected to an integrator that is triggered by the central control unit, so that the received response signal is only detected in predefined timing windows and is fed to the evaluation unit. In this context, the timing windows are selected by the triggering operation in accordance with the expected propagation delays of the partial signals of the individual arrays. This makes it possible to reliably distinguish among the arrays and, thus, among the individual Bragg grating sensors of the individual arrays.
Besides the feasibility of operating the optical fiber Bragg grating sensors in the reflection mode, it is also possible to operate them in the transmission mode. This proves to be especially advantageous when working with vast structures to be tested, since the cabling can be laid out in a simpler fashion.
It has proven to be especially worthwhile for the delay time selection to be controlled by a preferably central control unit. This makes it possible, in accordance with specific requirements, to adapt the variably selected delay times to the measuring tasks and the measuring conditions. In particular, it is possible to exclude individual arrays from the measurement by selecting a very long or infinitely long delay time, and to use only a few individual arrays. Moreover, individual arrays can be selectively switched on or off by altering the delay times during the measurement. This renders possible a very universal application of the various arrays, which are combined to form a total arrangement made up of a plurality of arrays. This arrangement is preferably controlled by a shared central control unit, so that any intervention in the measuring procedure or in the measuring method can be simply carried out.
It has proven to be especially beneficial to selectively choose the delay times of the delay elements, i.e., by properly selecting the delay elements, one is able to reliably distinguish among the reflected or transmitted response signals of the individual Bragg grating sensors. This can be achieved, in particular, by having the delay times always represent a multiple of one another, by satisfying the following rule, for example:
t1=a*ni 
t1 representing the delay time of the i-th array, n a natural number greater than =2, and a the length of the base time interval.
It is advantageous to provide one array that does not have a delay element, i.e., that has a delay time of 0 seconds. This simplifies the design of the arrangement.
Selecting delay times ti in this manner provides a reliable differentiation of the response signal with respect to differentiation of the partial response signals of the individual arrays.